Static dielectric constants of Ge, Sn, and isoelectronic semiconductor
s are calculated by using the full f-sum rule including all possible t
ransitions of d band electrons. The d band effect on the dielectric co
nstant is expressed by using a newly defined D factor different from t
he empirical D factor introduced by Van Vechten. The new D factor is d
irectly expressed in terms of the average band gap, the energy of the
d band, and the coupling oscillator strength between valence and d ban
ds so that it has a well defined meaning in contrast to the empirical
D factor. The calculated D values well reproduce the observed dielectr
ic constant and the effective number of valence electrons which repres
ents the excess deviation from the usual valence electron number.